Security officials said the base in the large but sparsely populated province of Hadramawt is supposed to be protected by several checkpoints leading to its main gate, but that no security was posted outside the military compound when the attack took place.

The military sent in reinforcements and troops are now surrounding the compound, intermittently clashing with the attackers, the officials and a Defense Ministry statement said.

The Ministry said it has managed to evacuate the building but other security officials say there are still an unknown number of officers and soldiers inside.

The attack underscores al-Qaida’s ability to exploit security lapses in Yemen, despite a dramatic rise in the number of U.S. drone strikes on militants there — including in Hadramawt — since President Barack Obama took office.

The group was blamed for an assault earlier this month that killed 38 soldiers in the nearby province of Shabwa.

In the Monday attack, authorities believe at least 15 militants stormed into the base on the eastern outskirts of the city of Mukalla.

The Defense Ministry statement said the militants rode in on two military vehicles, one of which was a car bomb that blasted through the main gate of the compound. They then gunned down security guards before taking over the facility after a brief firefight inside, officials said.

Officials have not released a death toll. The Defense Ministry statement said some of the attackers were killed and wounded, without giving specific numbers.

A colonel who works at the base but who was not there when it came under attack says he received phone calls from comrades inside who alerted him to the attack.

The officials and colonel spoke on condition of anonymity because they were not authorized to speak to the media.

For a time, al-Qaida in the Arabian Peninsula, as the group is known in Yemen, was perhaps the network’s most powerful offshoot in the region. In 2011, during political turmoil in Yemen, al-Qaida fighters temporarily seized control of a string of cities and towns in the south, where they remain active.